German Patent No. DE 35 36 820 C2 and the ISO standards 14819-1, -2 and -3 describes a method for the radio transmission of encoded traffic messages. The TMC (traffic message channel) method described therein follows the approach of disassembling conventional traffic messages into their elementary components, that is, particularly the location of an event, the direction of travel, the extent of the event and the event itself, of cataloging these components and assigning corresponding predefined codes to the catalogued components of the traffic messages. Instead of the traffic messages themselves, only the codes assembled in accordance with the message to be transmitted are then transmitted. For example, particularly important points along important traffic routes, that is, for example, entrances and exits, gas stations, rest areas etc. along expressways, are assigned so-called location codes. In a location table, these location codes are each assigned location names as well as references to locations preceding or following the respective location along the route or to their location codes. By transmitting such a location code and a travel direction, the location of a traffic-related event on a route section between two coded locations and a determinate travel direction are thus defined. So that only a limited address space needs to be made available for the location codes, different location tables are provided for the different countries, a particular country-specific location table being identifiable via an assigned location table number (LTN).
In the radio unit, the transmitted codes are assigned to corresponding message components with the aid of locally stored decoding tables and are subsequently shown on the display or are converted into spoken messages using language synthesis and output via the connected loudspeakers.
These TMC traffic messages are transmitted inaudibly along with a radio broadcast program via a radio frequency, for example, using the so-called radio data system (RDS), which is specified for example in DIN/EN 50 067.
RDS-TMC is currently implemented in two variants throughout Europe. On the one hand as a free service, which can be received in broad areas of Europe, on the other hand as a pay service (Pay TMC or Conditional Access (CA)), which is offered, among other places, in France, Great Britain and in parts of Germany. This service is often offered by private service providers and requires special software in the receivers so that these are able to decode the encrypted traffic messages. The providers additionally require that the terminal unit manufacturers pay a certain fee to the providers for each unit that is to be able to decode CA.